This proposal is aimed at establishing neural sites and cellular mechanisms by which steroids promote sex differences in vocal ability and neural organization. In zebra finches, song behavior is androgen-dependent and normally produced only by males. Estrogens act during development to determine the organization and extent of androgen accumulation within song regions. Androgens induce further neural growth in adulthood, and promote song development. This proposal focuses on three important issues regarding the sexual differentiation of the song system. The first regards where estrogen (E2) acts to masculinize developing song regions. Steroid autoradiography will be used to assess which developing song regions accumulate E2. Intracranial E2 implants will be used to determine which of these neural sites are primary targets for E2 action, and which are influenced indirectly, or transynaptically. The second issue concerns how early E2 exposure determines the extent of androgen uptake in the adult brain. Recent studies indicate that in one song region, early E2 exposure promotes the addition of androgen target cells during adolescence. Thymidine autoradiography will be used to determine if this cell addition involves neurogenesis, and whether E2 enhances either the proliferation or survival of cells in this region. The existence of a critical period for the estrogenic regulation of androgen accumulation will be determined by examining the pattern of androgen uptake in birds given E2 in adulthood. The third issue regards the neural sites and functional significance of androgen action in the adult brain. Using light microscopic analyses of cell morphology in sone regions, and behavioral analyses, the time course of androgen-induced nerual change will be assessed and related to vocal development. Projections of androgen-accumulating cells will be established through the simultaneous use of fluorescent retrograde tracers and steroid autoradiography, and intracranial antiandrogen implants will be used to determine if androgen-accumulating neurons in certain song regions mediate androgen-induced neural growth in their efferent targets. A better understanding of the sexual differentiation process holds great promise for therapeutic remedy of abnormalities in sexual differentiation in humans. It will also increase our understanding of cellular mechanisms that permit aspects of neural plasticity that are manifest in birds, yet less robust in humans.